Fugitive emissions are air pollution emissions that are not released from stacks designed as release points. Instead, fugitive emissions escape from industrial processes by means such as evaporation from wastewater treatment areas or leaks at process components. Leaks may occur at process components such as pumps, compressor seals, flanges, valves, pipe thread connections, and open ended lines on valves that are shut off. Rising concerns over hazardous air pollutants and greenhouse gases have led to the need for improved quantification of fugitive emissions to the environment.
Fugitive emissions are very difficult to quantify without expensive and time consuming measurement programs. Consequently, the oil, gas, and chemical industries typically use techniques to estimate the emission rate. These techniques are easy to apply but have several large uncertainties inherent in there use, as described below.
All emission measurement methods rely on determining the concentration of the compound(s) being emitted from the source and an estimation of the amount of dilution that takes place between the source and the point of concentration measurement. For example, when using an enclosure technique, the leaking component is wrapped with a nonpermeable material and a clean purge gas (such as nitrogen) sweeps through the enclosure at a measured flow rate. The known flow rate of purge gas provides a known dilution rate of the compound(s) leaking from the component. In the case of methane (CH.sub.4), the emission rate using an enclosure measurement can be calculated from the purge flow rate through the enclosure and the concentration of methane in the outlet stream as follows: EQU Q.sub.CH4 =F.sub.purge.times.C.sub.CH4.times.10.sup.-6
where:
Q.sub.CH4 =emission rate of methane from the enclosed component (ml/min), PA1 F.sub.purge =the purge flow rate of the clean air or nitrogen (ml/min), and PA1 C.sub.CH4 =the measured concentration of methane in the exit flow (ppm). PA1 C.sub.VOC =concentration read by VOC (ppm), PA1 Q.sub.LEAK =volumetric leak rate of methane from the component (ml/min), and PA1 Q.sub.VOC =volumetric sampling flow rate of air drawn into the VOC (ml/min).
The enclosure measurement technique is relatively accurate, but requires extensive time and effort to set up. Processes and components of concern must be carefully wrapped with the non-permeable material so that no unwanted gas or air enters the enclosure. The time and expense associated with this technique makes it prohibitive for routine monitoring of gas leaks.
Another technique to determine leak rate uses correlations which have been developed to relate the concentration measured near a leaking component to its actual leak rate. These relationships have been developed by correlating leak rates measured at components using the enclosure technique to the maximum concentrations measured at either 1 cm or 1 mm from the components using a portable volatile organic compound (VOC) analyzer. Correlations have been reported from work sponsored by the EPA (CMA, 1989) and the American Petroleum Institute (API) (Webb and Martino, 1992). These correlations essentially provide an empirical expression of the average dilution of the leaking material as it travels from the leak to the detector of the VOC. A plot of leak rate versus VOC screening value has been completed by others (CMA, 1989), and the scatter in measurements was demonstrated to be three orders of magnitude.
A theoretical ideal correlation between emissions and the resulting portable VOC analyzer screening concentration can be determined for a given VOC. This correlation depends on the volumetric sampling flow rate which is drawn into the instrument. If a methane leak is entirely captured by the instrument during screening, the concentration measured by the instrument will be: ##EQU1##
where:
For instance, one commonly used instrument for screening components for fugitive emissions draws a nominal sample flow of 1000 ml/min. If a leak rate of 10 ml/min is entirely captured during screening, the resulting concentration will be 10,000 ppm, or 1%. Similarly, a leak rate of 1 ml/min would result in a VOC concentration of 1000 ppm, or 0.01%.
In practice, the actual ideal concentration is rarely achieved because the leak is not completely captured. The amount of the leak which is captured using this technique can vary significantly and is affected by the following: the sampling flow rate; the distance of the sampling probe from the leak; the ambient wind speed or air movement; and the characteristics of the leak such as its velocity upon leaving the component and the area over which the leak occurs. Larger sampling distances and increased ambient wind speeds reduce the influence of the sample flow on the air movement around the leak and give the plume from the leak more opportunity to diffuse away from the probe and avoid capture. For larger leaks, the plume may have enough momentum to overcome the flow field generated by the sampling flow. For leaks which escape from several points around a component, the area covered by the sampling probe flow rate may not be large enough to capture the plume.
For the foregoing reasons, there is a need for less time consuming, yet precise methodology for measuring fugitive air emissions. An instrument for performing this new methodology should allow for rapid, accurate and reliable quantification of emission concentrations from processes and components.